This invention relates in general to techniques for improving bandwidth of devices having a stripline to slotline transition and, more particularly, to techniques for improving bandwidth of antenna elements having a stripline to slotline transition.
Various different types of devices include a stripline to slotline transition, one example of which is an antenna element configured for use in a phased array antenna. In one form of such an antenna element, a balun is provided to improve the bandwidth of the antenna element, and communicates with the end of the slotline nearest the region of the stripline to slotline transition.
In the most common existing antenna element of this type, the balun has a predetermined configuration which does not change during normal operation of the antenna element. This type of balun provides good electrical performance within a limited bandwidth, but suffers from electrical performance degradation when used over a relatively wide bandwidth. It is desirable to be able to provide an antenna element which has a relatively wide 10:1 bandwidth, commonly referred to as a xe2x80x9cdecadexe2x80x9d bandwidth. However, known balun structures that can be used to achieve a decade bandwidth have a relatively large physical size. In an array antenna, the spacing between adjacent antenna elements becomes progressively smaller as the highest operational frequency of the antenna increases. Consequently, the space available for each antenna element, often called the unit cell size, is so small in comparison to low frequency wavelengths that the known balun structure capable of providing a decade bandwidth is too big to fit within the unit cell size.
From the foregoing, it may be appreciated that a need has arisen for a balun structure which is suitable for use with a stripline to slotline transition and which avoids some or all of the disadvantages of pre-existing arrangements.
According to one form of the present invention, a method and apparatus are provided to address this need, and involve operation of an apparatus which includes a conductive section having a recess with a balun portion and an elongate slot portion communicating at one end with the balun portion, and which includes an elongate conductive element extending generally transversely with respect to the slot portion in the region of the one end thereof. The technique includes selectively operating a capacitive element in one of first and second modes in which the capacitive element respectively has first and second capacitances that are substantially different, the capacitive element having first and second terminals which are respectively coupled to the conductive section at spaced first and second locations disposed along a periphery of the balun portion, wherein in the first mode the balun portion and the capacitive element collectively have a first effective impedance, and in the second mode the balun portion and the capacitive element collectively have a second effective impedance different from the first effective impedance.
According to a different form of the present invention, a method and apparatus involve operation of an apparatus which includes a conductive section having a recess with a balun portion and an elongate slot portion communicating at one end with the balun portion, the balun portion having in a radio frequency band an effective impedance which is substantially larger than an effective impedance of the slot portion, and which includes an elongate conductive element which extends generally transversely with respect to the slot portion in the region of the one end thereof. The technique involves selectively operating a switch in one of first and second modes in which the switch respectively is substantially transmissive and substantially non-transmissive to radio frequency signals, the switch having first and second terminals which are respectively coupled to the conductive section at spaced first and second locations disposed along a periphery of the balun portion, wherein in the first mode the balun portion and the switch collectively have a first effective impedance and in the second mode the balun portion and the switch collectively have a second effective impedance different from the first effective impedance, the first and second effective impedances each being substantially larger than the effective impedance of the slot portion.